'\" et
.TH PUTENV "3P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\"
.SH PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
.\"
.SH NAME
putenv
\(em change or add a value to an environment
.SH SYNOPSIS
.LP
.nf
#include <stdlib.h>
.P
int putenv(char *\fIstring\fP);
.fi
.SH DESCRIPTION
The
\fIputenv\fR()
function shall use the
.IR string
argument to set environment variable values. The
.IR string
argument should point to a string of the form "\c
.IR name =\c
.IR value \c
\&".
The
\fIputenv\fR()
function shall make the value of the environment variable
.IR name
equal to
.IR value
by altering an existing variable or creating a new one. In either
case, the string pointed to by
.IR string
shall become part of the environment, so altering the string shall
change the environment.
.P
The
\fIputenv\fR()
function need not be thread-safe.
.SH "RETURN VALUE"
Upon successful completion,
\fIputenv\fR()
shall return 0; otherwise, it shall return a non-zero value and set
.IR errno
to indicate the error.
.SH ERRORS
The
\fIputenv\fR()
function may fail if:
.TP
.BR ENOMEM
Insufficient memory was available.
.LP
.IR "The following sections are informative."
.SH EXAMPLES
.SS "Changing the Value of an Environment Variable"
.P
The following example changes the value of the
.IR HOME
environment variable to the value
.BR /usr/home .
.sp
.RS 4
.nf

#include <stdlib.h>
\&...
static char *var = "HOME=/usr/home";
int ret;
.P
ret = putenv(var);
.fi
.P
.RE
.SH "APPLICATION USAGE"
The
\fIputenv\fR()
function manipulates the environment pointed to by
.IR environ ,
and can be used in conjunction with
\fIgetenv\fR().
.P
See
\fIexec\fR()
for restrictions on changing the environment in multi-threaded
applications.
.P
This routine may use
\fImalloc\fR()
to enlarge the environment.
.P
A potential error is to call
\fIputenv\fR()
with an automatic variable as the argument, then return from the
calling function while
.IR string
is still part of the environment.
.P
Although the space used by
.IR string
is no longer used once a new string which defines
.IR name
is passed to
\fIputenv\fR(),
if any thread in the application has used
\fIgetenv\fR()
to retrieve a pointer to this variable, it should not be freed by calling
\fIfree\fR().
If the changed environment variable is one known by the system (such as
the locale environment variables) the application should never free the
buffer used by earlier calls to
\fIputenv\fR()
for the same variable.
.P
The
\fIsetenv\fR()
function is preferred over this function. One reason is that
\fIputenv\fR()
is optional and therefore less portable. Another is that using
\fIputenv\fR()
can slow down environment searches, as explained in the RATIONALE
section for
.IR "\fIgetenv\fR\^(\|)".
.SH RATIONALE
Refer to the RATIONALE section in
.IR "\fIsetenv\fR\^(\|)".
.SH "FUTURE DIRECTIONS"
None.
.SH "SEE ALSO"
.IR "\fIexec\fR\^",
.IR "\fIfree\fR\^(\|)",
.IR "\fIgetenv\fR\^(\|)",
.IR "\fImalloc\fR\^(\|)",
.IR "\fIsetenv\fR\^(\|)"
.P
The Base Definitions volume of POSIX.1\(hy2017,
.IR "\fB<stdlib.h>\fP"
.\"
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1-2017, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition,
Copyright (C) 2018 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
In the event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
.PP
Any typographical or formatting errors that appear
in this page are most likely
to have been introduced during the conversion of the source files to
man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
